Re: [Qemu-devel] [RFC PATCH RDMA support v4: 08/10] introduce QEMUFileRDMA

["Michael R. Hines" <mrhines@linux.vnet.ibm.com>]

On 03/19/2013 10:22 AM, Paolo Bonzini wrote:
> Il 19/03/2013 15:10, Michael R. Hines ha scritto:
>> On 03/19/2013 09:45 AM, Paolo Bonzini wrote:
>>> This is because of downtime: You have to drain the queue anyway at the
>>> very end, and if you don't drain it in advance after each iteration, then
>>> the queue will have lots of bytes in it waiting for transmission and the
>>> Virtual Machine will be stopped for a much longer period of time during
>>> the last iteration waiting for RDMA card to finish transmission of all
>>> those
>>> bytes.
>>> Shouldn't the "current chunk full" case take care of it too?
>>>
>>> Of course if you disable chunking you have to add a different condition,
>>> perhaps directly into save_rdma_page.
>> No, we don't want to flush on "chunk full" - that has a different meaning.
>> We want to have as many chunks submitted to the hardware for transmission
>> as possible to keep the bytes moving.
> That however gives me an idea...  Instead of the full drain at the end
> of an iteration, does it make sense to do a "partial" drain at every
> chunk full, so that you don't have > N bytes pending and the downtime is
> correspondingly limited?


Sure, you could do that, but it seems overly complex just to avoid
a single flush() call at the end of each iteration, right?

> If there is no RAM migration in flight.  So you have
>
>     migrate RAM
>     ...
>     RAM migration finished, device migration start
>     put_buffer <<<<< QEMUFileRDMA triggers drain
>     put_buffer
>     put_buffer
>     put_buffer
>     ...

Ah, yes, ok. Very simple modification......